Over the years, the television tube has evolved to the point where it remains the source that provides the best overall picture quality. In the field of medical imaging cathode ray tubes (CRTs) offer tremendous advantage due to their superior dark level and speed of response which permit their use as a 3-D display. Also the low cost of CRTs compared to the other technologies makes them competitive, particularly when approaching the highest standards for high definition television. In spite of these advantages, CRTs are losing in the marketplace as television displays and projectors based on liquid crystal concepts, e.g., liquid crystal displays (LCDs), micro-mirror digital light processors, and plasmas have evolved in recent years. Similarly, field emission displays (FEDs), a breakthrough in imaging technology, have fallen out of favor in comparison to liquid crystal displays (LCDs) and plasma display technologies.
There have been substantial improvements in the development of all the primary components in the CRT/FED except for the cathodoluminescent phosphor. Improved phosphors could be applied to cathode ray tubes (CRTs) and field emission displays (FEDs) so that light output from these devices can be increased accordingly and serve to provide a much brighter display. The limitation of CRT and FED technologies arises from the current cathodoluminescent phosphors used in these devices. These phosphors limit the display brightness due to their low output, reduced contrast due to their amorphous structure, and need for high voltage across the tube for their operation.
The FED operates by a video controlled array of micro beams so as to simultaneously project electrons across a narrow space to impact a cathodoluminescent phosphor causing light emission and display. The concept for the FED caused substantial excitement because there was a very real potential for creating a truly superior image at reasonably cost. Also, the United States Government has invested substantial amounts of money in FED development since it represented not only a breakthrough in imaging technology, but also because it created an opportunity to bring the manufacture of video displays back to the United States. As with CRTs there have been substantial improvements in the development of all the primary components in the FED except for the cathodoluminescent phosphor. Improvements in the technology are needed or are required to improve the production of a bright image with a lower energy beam. Currently, an energy of at least 10 keV is required to produce an image for even a limited number of device applications.
In the medical diagnostics field “Totally Digital” and “Film-less Radiology” is rapidly replacing the conventional film-screen based radiology (Henri et al., J. Digit. Imaging 12:178-180, 1999; Hayt et al., J. Digit Imaging 14:62-71, 2001; Huang, Ann. Intern. Med. 112:203-220, 1990; Blume, J. Digit. Imaging 12:43-47, 1999). Digital imaging sensors and digital displays are used instead of the traditional sensor and display, typically comprising the film-screen combination and the associated film-light box. In digital x-ray imaging systems, the functions of image detection and image display are separated and the images can be presented to the human observer at optimum information transfer, i.e., after contrast and spatial frequency response enhancement. In fact, the availability of efficient software permits presentation of the information according to a display function standard such that the images appear practically at identical gray scales anywhere the same or similar display software is used. This fidelity of image presentation increases the consistency and accuracy of diagnosis. However, as advanced as the acquisition devices are, displaying these digital images still presents a challenge. Presently the CRT is considered the most mature electronic display available, thus it is the primary candidate to display digitally acquired radiographs and mammograms, despite the fact that its performance is still not adequate. Most high performance CRTs can display images with matrix sizes of 2048×2560. An advantage of CRTs is their superior dark level, which cannot be matched by that of LCDs. Another advantage of CRTs is their high response speed, which permits their use for 3-D information and dynamic displays. Due to their slow response, LCDs cannot perform this function as efficiently. Nevertheless LCDs compete with CRTs in the market place.
Usually the environment in a radiograph reading room is characterized by subdued lighting, but the performance of the radiologist is improved by high maximum display luminance. Typical maximum luminance values range around 400 cd/m2, however recent efforts are being directed toward achieving a maximum luminance of about 2100 cd/m2. Furthermore, in countries like Japan, radiologists typically read radiographs in brightly lit rooms, which require displays reaching even higher maximum luminance values. Thus, a need exists for higher luminance displays.
Bright scintillators are also needed for imaging ionizing radiation. The charged particle and x-ray/gamma-ray imaging community is particularly interested in new fast scintillators with high density and high light output for applications in nuclear medicine such as single photon emission tomography (SPECT) (Korzhik and Lecoq, “Search of New Scintillator Materials for Nuclear Medicine Applications”, IEEE Nuclear Science Symposium-Medical Imaging Conference, Lyon, France, 2000), computed tomography (CT), diagnostic x-ray imaging (Qu et al., “A Search for a New Type of Lead Tungstate with High Light Yield”, IEEE Nuclear Science Symposium-Medical Imaging Conference, Lyon, France, 2000; Balcerzyk et al., “Search for Indium and Thallium based High Density Scintillators, IEEE NSS-MIC Conference, Lyon, France, 2000). Many important applications such as macromolecular crystallography and high-speed imaging are “light starved” and need converters with significantly higher light yield (output) than is currently possible. Also, many applications require that integrating detectors (such as a charge-coupled device (CCD) or a hydrogenated amorphous Silicon (a-Si:H) flat panel array) detect a single x-ray or gamma-ray photon. While it is possible to achieve such a sensitivity using very highly sophisticated CCD arrays, they are prohibitively expensive. Higher light output would improve the image signal-to-noise ratio (SNR) and provide images with superior quality than are currently possible.
It is known that a-Si:H flat panel detectors have a problem of read noise. Noise can not be reduced, so the common approach is to attempt to increase the signal to maintain a high SNR. To achieve this, the current thinking is to introduce a signal amplification stage within the flat panel. This is not only expensive, as new fabrication lines will have to be developed, but it is not yet known if the additional electronics will introduce additional noise. Also, there are concerns about the dynamic range of such a device. Higher light output obviates these problems.
The methods and devices of the present invention provide for an increased light output or luminescence that solves many of the current deficiencies in the scintillators used in video displays, as well as radionuclide and x-ray imaging devices.